Beam 1a noisy 01nov06

10nov06

The problem:

The alfalfa a2010 project reported extra noise in the
drifts done with beam1a on 01nov06. They are doing drift scans using 100
Mhz bw centered at 1385 Mhz. They are dumping at a 1 second rate with 4096
channels per spectra. Each strip lasts 600 seconds.

Plotting the total power jumps and ripple strength:

Average bandpass 7 beams: This has been averaged over the 600 seconds.
Black is polA and red is polB. The dashed green lines are the portion of
the spectrum used to measure the ripple strength.

Total power vs time for 7 beams: The total power for each spectra
was computed (excluding the regions with rfi). The total power time series
was normalized to the median value for each beam and then plotted
versus time. Black is polA and red is polB. The bottom trace is beam0 while
the top trace is beam6.

The positive going bumps are continuum sources.

Pol A and polB should track each other. Beams 2,4,5, and 6 have some variation
in the polA/B ratio with time.

Beams 0A and 1A are noisier than the others. Beam 1A (reported as bad)
is the worse.

Rms/Mean for each frequency channel: The rms/mean was computed for
each frequency channel over the 600 time samples. Outliers (continuum sources)
were not included in the rms computation. The green dotted line is
the expected rms (=1.2(3lvl)/sqrt(100e6/4096*1.2(sinx/x width)*1.)). The
rms vs frequency for beam 1a shows a frequency dependence (not seen in
the others beams).

Blowup of avg bandpass showing ripple: A 1st order polynomial and
a 9th order harmonic was removed from the 100 Mhz average bandpass for
each beam. 5 Mhz (1360 to 1365 MHz) was then plotted for each beam. Black
is polA and red is polB. The y axis is in units of Tsys.

You can see a high frequency ripple in beams: 1A,2A,3A,3B, and 4A. This
is the 174 Khz ripple caused by the reflections in the fiber optics cable.
This was seen in 07sep04 in the galfa data and again in 11jul05
in the alfalfa data.

Back in jun05 pixel had the largest ripple (about .003 tsys). We replaced
the fiber optic receiver at the time. The other pixels had ripple on the
order of 0 to .002 Tsys.

Avg ACF showing strength of reflection: For each of the 600 spectra:

The frequency range 1352 to 1418 Mhz was extracted and a 9th order harmonic
was removed.

Each 1 second spectra was then normalized so that its median value was
unity.

For each 1 second spectra the acf was computed.

All 600 acf's were averaged and plotted.

Each beam is color coded. PolA is always the lower trace of each
pair. The x axis is in useconds (lag delay). The y axis is in units of
Tsys (with offsets for plotting).

The birdie at 5.782 useconds is about the time needed for a reflection
to go up and back in the fibers.

Beam1a has the largest ripple (.002 Tsys) but it is not much larger than
some of the other pixels. It has the same order of magnitude as the values
measured back in jun05.

Stability of the reflection: For each 1 second acf:

the peak value was recorded for the 5.782 usec delay (actually the peak
was taken from 5 lags around this value).

The amplitude of the ripple was then plotted versus time. Pol A is black
and polB is red. The vertical scale is in units of Tsys.

The amplitude of the ripple was stable for all beams except beam1A.

Beam1a would jump between .001 Tsys an .005 Tsys.

The ripple should be a function of the power reflecting at each end of
the fibers. Since we normalized each 1 second spectra to unity, there should
be no power variation in the ripple power (since the total power was forced
to be constant). This means that either the reflection coefficient is changing
or the optical transmitter if jumping on time periods of a second or more.

Conclusions:

Extra noise was seen in beam1A of alfa on 01nov06.

The total power versus time shows beam1A to be worse than the other beams
(beam0a is also worse than the others).

The rms/mean by channel shows a frequency dependence for beam1A. This means
the jumps are not constant in frequency.

The 174Khz ripple caused by reflections in the fibers is present in a few
beams. The amplitude is similar to what is was back in 2004 and 2005 (except
for beam1a).

The amplitude of the reflection in beam1a is changing in time from .001
Tsys to .005 Tsys. The jumps happen on orders of a second.

The jumps are not caused by changes in total power (since each spectra
was normalized to unity). This probably means that the fiber optic transmitter
is failing. A less likely cause could be that the reflection coefficient
is changing on time scales of a second.